Conventional electrophotographic toner powders are made up of a binder polymer and other ingredients, such as pigment and a charge control agent, that are melt blended on a heated roll or in an extruder. The resulting solidified blend is then ground or pulverized to form a powder. Inherent in this conventional process are certain drawbacks. For example, the binder polymer must be brittle to facilitate grinding. Improved grinding can be achieved at lower molecular weight of the polymeric binder. However, low molecular weight binders have several disadvantages; they tend to form toner/developer flakes; they promote scumming of the carrier particles that are admixed with the toner powder for electrophotographic developer compositions; their low melt elasticity increases the off-set of toner to the hot fuser rollers of the electrophotographic copying apparatus, and the glass transition temperature (Tg) of the binder polymer is difficult to control. In addition, grinding of the polymer results in a wide particle size distribution. Consequently, the yield of useful toner is lower and manufacturing cost is higher. Also the toner fines accumulate in the developer station of the copying apparatus and adversely affect the developer life.
The preparation of toner polymer powders from a preformed polymer by a chemically prepared toner process such as “Evaporative Limited Coalescence” (ELC) offers many advantages over the conventional grinding method of producing toner particles. In ELC, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent. The resultant particles are then isolated, washed and dried.
In the practice of this technique, polymer particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by mechanical shearing using rotor-stator type colloid mills, high pressure homogenizers, agitation, etc.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of polymer particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al., and U.S. Pat. No. 6,294,595 to Tyagi, incorporated herein by reference for all that they contain.
This technique includes the following steps: mixing a polymer material, a solvent and optionally a colorant and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent and washing and drying the resultant product.
There is a need to reduce the amount of toner applied to a substrate in the Electrophotographic Process (EP). Porous toner particles in the electrophotographic process can potentially reduce the toner mass in the image area. Simplistically, a toner particle with 50% porosity should require only half as much mass to accomplish the same imaging results. Hence, toner particles having an elevated porosity will lower the cost per page and decrease the stack height of the print as well. The application of porous toners provides a practical approach to reduce the cost of the print and improve the print quality.
U.S. Pat. Nos. 3,923,704; 4,339,237; 4,461,849; 4,489,174 and EP 0083188 discuss the preparation of multiple emulsions by mixing a first emulsion in a second aqueous phase to form polymer beads. These processes produce porous polymer particles having a large size distribution with little control over the porosity. This is not suitable for toner particles.
U.S. Publication No. 2005/0026064 describes a porous toner particle. However, control of particle size distribution is a problem and these porous particles have porous surfaces. Conventional toners have solid surfaces and properties such as tribocharging and transfer may be adversely affected by a porous surface. The present invention solves these problems and provides a less complex method to manufacture porous particles.
U.S. Pat. Nos. 5,608,017 and 5,717,041 describe a polymerized particle useful as toner having a cavity structure. However, FIG. 3 in said patents show that the cavities connect to the particle surface making it porous.
U.S. Pat. No. 4,379,825 describes porous toners made by mixing and kneading a polymeric material including an elimination compound. The toner has voids or pores on the surface.
Japanese Kokai 63-147171 describes a developer suitable to a development system constituted by combining the advantages of a wet and a dry system where a small-diameter sponge is impregnated with a liquid developer. Said small-diameter sponge has a porous surface.
Japanese Kokai 08-220793 describes electrophotographic toner where porosity of the toner particle is specified to 0.51 to 0.54. However, there is no mention of a non-porous shell.
Japanese Kokai 01-167846 describes a toner that is formed by impregnating liquid ink in the pores of microporous polymers particles. A porous surface is required to impregnate the ink.
An object of the present invention is to provide a polymer particle with porosity.
A further object of the present invention is to provide a toner particle with porosity.
A still further object of the present invention is to provide a toner particle with a narrow size distribution.
A still further object of the present invention is to provide a porous toner particle with surface properties similar to solid toner particles.